Boo Radley serves as a physically absent, yet legendary presence for much of To Kill a Mockingbird. As the subject of much public scrutiny and gossip, most of Maycomb chooses to perpetuate the mythology around Boo: that Boo once stabbed his father with a pair of scissors and has since then chosen to prowl the town at night, feasting on small animals and lurking in shadows.
In reality, Boo was severely punished by his father as a child; after a prank went wrong, Boo's father had him locked up in the house, an isolation which lasted for fifteen years. This abuse--as well as the cruelty of the townspeople of Maycomb--turned Boo into a reclusive man. Regardless, he is still an innocent, gentle soul who expresses curiosity about the children who play around his yard. Boo secretly does small acts of good throughout the novel, from mending and folding Jem's lost trousers to leaving chewing gum in the "gift" tree for the siblings. Ultimately, his motivation is to protect the children, and he ends up breaking his seclusion in order to rescue Jem and Scout from the murderous attack of Bob Ewell on Halloween night.
Overall, Boo is one of the "mockingbirds" references by the book's title--an innocent creature who is sinfully destroyed by the cruelty of others.
Although Boo Radley is initially presented by the children as a rather malevolent character because he stabbed his father with the scissors, he does not appear sinister in his actions. We find out that Boo's troubles began with a minor infraction that was punished as a major crime and he was isolated from his peer group for a long time. Boo seems to respond to the interest of the children by reaching out to them in friendship by leaving them presents in the tree, repairing Jem's pants, and finally saving Jem and Scout from Bob Ewell at the end of the book. He is protective of the children. Perhaps he sees his own innocence in them. He is a grown man physically but socially and psychologically he is as much of a child as Jem and Scout.